1. Field of the Invention
This invention relates to sealing elements used between reciprocal tubular elements and, more particularly, to nonelastomeric seal cartridges used with valve actuators responsive to control fluid pressure in oil and gas well operations.
2. Description of the Prior Art
Dynamic seal systems have been conventionally employed between reciprocal tubular members in a variety of applications in oil and gas well operations. Typically, packing or sealing elements disposed between two tubular members or conduits comprise O-ring seals, T-seals, and multi-element chevron seal stacks. For oil and gas wells in which fluids are produced at surface and subsurface temperatures on the order of 250.degree. F. and in which corrosion is not significant, conventional elastomeric seals may be used to satisfactorily maintain sealing integrity between the reciprocal tubular members. These elastomers, such as nitrile, are deformable under low stress and when the stress is released will return to their original configuration under the temperatures and pressures previously encountered in producing oil and gas wells. However, these materials will lose their resiliency and are incapable of maintaining adequate sealing integrity under the more hostile environments. Conventional elastomeric seals lose their inherent resiliency and their ability to establish sealing integrity between adjacent tubular members. For example, at temperatures in the order of 450.degree., conventional elastomers cannot be used for sealing applications in oil and gas wells.
In addition to the effects of temperature in diminishing the performance of conventional elastomers, corrosion inhibitors used with the sour, highly corrosive fluids may react with conventional elastomers rendering the conventional elastomers completely unsuitable. For example, amine inhibitors used to overcome the damaging effects of hydrogen sulfide contained in sour fluid wells can render many conventional elastomers completely useless for sealing and packing applications.
The performance of conventional elastomers at elevated temperatures is also detrimentally effected by gas impregnation of the elastomeric sealing elements. These conventional elastomers are not impervious to gas, and during operation in oil and gas well environments, gases tend to impregnate and indeed pass directly through the sealing elements. The gases impregnating the elastomeric sealing elements expand when heated. The heated gases apply stresses to the elastomer and tend to damage the elastomeric elements when heated. Indeed, when these seals are removed from the concentric tubular members confining the seals, the impregnated gases may completely rupture and destroy the sealing element. This phenomenon is known is an explosive decompression of the seals.
Sealing elements fabricated using nonelastomer materials, such as polytetrafluoroethylene, commonly identified by the Dupont trademark Teflon, and polyphenolene sulfide, commonly referred to by the Phillips trademark Ryton, have been used in seal systems in oil and in gas applications. Nonelastomeric elements fabricated from these materials have been used in multi-component seal stacks as backups for new elastomers capable of operating under hostile conditions and in the presence of damaging fluids. These elastomers are, however, quite expensive. The use of completely nonelastomeric seal systems is therefore highly desirable.
Nonelastomeric seal systems cannot rely upon any inherent resiliency to energize the seal to maintain sealing integrity between adjacent surfaces. One means of imparting energy to a nonelastomeric seal stack is to employ an axially compressible spring member which continuously applies axial compression to the seal stacks to urge nonelastomeric seal elements into a radially expanded configuration. For example, an axially compressive spring can be used in conjunction with a plurality of chevron-shaped sealing elements to urge convex surfaces of the chevron-shaped sealing elements outwardly to maintain sealing integrity with concentric tubular members. Such axially compressive springs can also be used to maintain a force on nonelastomeric members, such as polytetrafluoroethylene, with a tendency to flow or plastically deform under pressure. An accumulative axially compressive spring will continue to apply compressive loads after plastic deformation of such members. Axially compressive springs are not, however, completely satisfactory, and other means of maintaining a seal using nonelastomeric sealing elements are needed for a satisfactory seal system. For example, axially compressive springs can be difficult to package in a multi-component seal system.
Seal stacks having better performance under hostile conditions than can be obtained with conventional elastomers are especially needed for valve actuators used in well safety systems and as components in natural gas transmission lines. Conventional actuators are utilized to manipulate a valve mechanism within a flow line into open and/or closed position in response to control pressure variation. Normally, these actuators comprise a shaft and a fluid activated mechanism in association with the shaft which, upon an increase in control fluid pressure causes longitudinal movement of the tubular shaft to shift a valve member in relation to the valve seat. Then the control line pressure produces the pressure force acting on the tubular shaft, thus permitting the valve to return to its unactuated position. Flapper or ball valves are frequently utilized in safety systems used in conjunction with the drilling, completion and production of offshore, as well as onshore, oil and gas wells. Additionally, actuating components can be utilized in natural gas transmission lines and in similar applications.